1. Technical Field of the Invention
The present invention relates to a method for driving solid-state image pickup devices.
2. Description of the Prior Art
When a color VTR (video tape recorder) camera for an interlace signal for the standard TV receiver is used for a solid-state image pickup devices such as a electronic still camera, or an image input device for personal computers, a signal processing for converting the pixel number and the scanning format is required, because the progressive format for monitors of personal computers is different from the interlace format.
Therefore, the progressive format solid-state image pickup devices which can read out all the pixels without the conversion processes are employed for the electronic still camera or the input device for the personal computers.
Nevertheless, the interlace format solid-state image pickup devices are used often, because they can be manufactured by fewer steps, their image cells can be highly integrated, and they can be used also as the color VTR camera, as described in TAKEMURA Hiroo “CCD Camera Technique” Radio Gijutu Co., Showa 61 (1986) Nov. 3, First Edition, pp 23–30, pp46–50.
There is shown in FIG. 7 a plan view of a conventional interlace solid-state image pickup device with a vertical charge transfer unit wherein signal charges are transferred by double layered electrodes and four phased pulses.
The conventional interlace solid-state image pickup device as shown in FIG. 7 comprises photo-electric conversion unit 101, vertical charge transfer unit 102, horizontal charge transfer unit 103, and output circuit 104. Concretely, one step vertical charge transfer unit 102 is connected with two horizontal photo-electric conversion units 101, or in other words, 1/2 step vertical charge transfer unit 102 is connected with one horizontal photo-electric conversion units 101.
There is shown in FIG. 8 a plan view of enlarged cells which comprises photoelectric conversion unit 101, vertical charge transfer unit 102, first charge transfer electrode 105, and second charge transfer electrode 106.
There is shown in FIG. 9 a cross sectional view of the cells along the lines I-I′ of FIG. 8. The cell as shown in FIG. 9 comprises N− semiconductor substrate 107, P− semiconductor substrate 108, N semiconductor region 109, P+ semiconductor 110, first charge transfer electrode 105 of first poly-silicon 111, second charge transfer electrode 106 of second poly-silicon 112, shading film 113 such as aluminum film, insulating film 114, and cover insulating film 115.
The conventional interlace solid-state image pickup device operates under the timing chart as shown in FIG. 10.
Firstly, in order to reset the charges in photo-electric conversion unit 101 at t1, a reverse bias voltage VHsub is applied to N−semiconductor substrate 107 as shown in FIG. 11. Hereupon, the charges in photo-electric conversion unit 101 are swept out into N−semiconductor substrate 107, because N semiconductor region 109 and P−semiconductor region 108 become complete depletion layers. Such an structure is generally called a vertical over-flow drain (OFD) structure as described in Journal of Television Society Vol. 37, No.10 (1983) pp782–787.
Next, a voltage VBsub is applied to N− semiconductor substrate 107 to start storing signal charges corresponding to the incident light, while surplus charges which can not be stored in photo-electric conversion unit 101 are excluded into N−semiconductor substrate 107 by using vertical OFD. Such an exclusion of the surplus charges is called blooming control.
Next, at the moment t2 when a prescribed exposure time (t1−t2) passes, the incident light is cut off by a cut off means such as a mechanical shutter which is positioned in front of the solid-state image pickup device.
Then, at the time t4, signal charges, for example, such as signal charges 11,12,13,31,32,33,51,52,53 in photo-electric conversion unit 101 are read out into vertical charge transfer units 102 which transfer vertically the signal charges line by line into horizontal charge transfer unit 103 which transfers the signal charges horizontally to output them from output circuit 104.
Finally, at the time t5, signal charges, for example, such as signal charges 21,22,23,41,42,43,61,62,63 in photo-electric conversion unit 101 are read out into vertical charge transfer units 102 and then outputted likewise from output circuit 104. Thus, the signal charges from all of the pixels for one frame of display can be acquired, as described in TAKEMURA Hiroo “CCD Camera Technique” Radio Gijutu Co., Showa 61 (1961) Nov. 3, First Edition, pp23–30, pp46–50.
However, the above-mentioned conventional device has a disadvantage that the read out saturation signal decreases with increasing read out cycles wherein the photo-electric conversion unit is read out several times part by part.